Aluminum alloy plate having excellent moldability and bake hardening properties

ABSTRACT

The purpose of the present invention is to provide an aluminum alloy plate capable of having a 0.2% proof stress during molding of no more than 110 MPa and a 0.2% proof stress after BH of at least 170 MPa. The present invention pertains to an aluminum alloy plate including, in mass %, 0.2%-1.0% Mg and 0.2%-1.0% Si, fulfilling {(Mg content)+(Si content)}≤1.2%, having a 20-50 μW/mg high exothermic peak within a temperature range of 230-330° C. in a differential scanning calorimetry curve, and having both excellent moldability and excellent bake hardening properties.

TECHNICAL FIELD

The present invention relates to an Al—Mg—Si alloy sheet. The aluminumalloy sheet referred to in the present invention means an aluminum alloysheet that is a rolled sheet such as a hot rolled sheet or a cold rolledsheet and has been subjected to refining such as a solution heattreatment and a quenching treatment, but is not yet subjected to a pressforming and a bake hardening treatment. Further, aluminum is hereinafteralso referred to as Al.

BACKGROUND ART

In recent years, because of environmental awareness and the like, thesociety's requirement for weight reduction in a vehicle such as anautomobile has been steadily increasing. In order to respond to suchrequirement, as a material for a large body panel structure (an outerpanel or an inner panel) of an automobile instead of a steel materialsuch as a steel sheet, application of an aluminum alloy materialexcellent in formability and bake hardenability and lighter in weighthas been increasing.

Among the large body panel structure of an automobile, for an outerpanel (outer sheet) such as a hood, a fender, a door, a roof, or a trunklid, use of an Al—Mg—Si-based AA or JIS 6000-series (hereinafter, alsosimply referred to as a 6000-series) aluminum alloy sheet, as a thin andhigh strength aluminum alloy sheet, has been studied.

The 6000-series aluminum alloy sheet contains Si and Mg as essentialcomponents. In particular, a 6000-series aluminum alloy with excess Sihas a composition in which the Si/Mg mass ratio is 1 or greater, and hasexcellent age hardenability. Because of this, formability for pressforming or bending into the outer panels of automobiles is secured bylowering the proof stress. In addition, it has such bake hardenability(hereinafter referred to also as BH response) that it undergoes agehardening upon heating in an artificial aging (hardening) treatmentperformed at a relatively low temperature, such as the baking treatmentof formed panels, and hence improves in proof stress, thereby ensuringthe strength required as a panel.

On the other hand, as is known well, an outer panel of an automobile ismanufactured by applying combined formings, such as stretch forming orbending forming in press forming, to an aluminum alloy sheet. Forexample, in a large outer panel such as a hood or a door, the shape of aformed product is made as an outer panel by press forming such asstretching, and then joining with an inner panel is executed by hem work(hemming) of a flat hem and the like of the outer panel peripheralsection to be formed into a panel structural body.

Here, the 6000-series aluminum alloy had an advantage of havingexcellent BH response, but had a problem of having aging properties atroom temperature, that is, of age hardening during retention at roomtemperature after solution heat treatment and quenching treatment toincrease the strength, thereby deteriorating formability into a panel,particularly the bendability. For example, in a case where a 6000-seriesaluminum alloy sheet is to be used for an automobile panel, it is placedat room temperature (standing at room temperature) for approximately 1month after the solution heat treatment and the quenching treatment(after manufacturing) at an aluminum manufacturer until forming into apanel at an automobile manufacturer, and comes to be significantly agehardened (room-temperature aged) during that time. Particularly, in theouter panel subjected to severe bending, there was such a case that,although forming was possible without any problem immediately aftermanufacturing, cracking occurred in hem working after the lapse of 1month. Therefore, in the 6000-series aluminum alloy sheet for anautomobile panel, particularly for an outer panel, it is necessary tosuppress room-temperature aging over a comparatively long period ofapproximately 1 month.

Moreover, in the case where such room-temperature aging is great, therealso is a case that the BH response deteriorate and the proof stress isnot improved to the strength required as a panel by heating during anartificial aging (hardening) treatment at a comparatively lowtemperature, such as a bake treatment and the like of the panel afterforming described above.

Hereto, in order to cope with such decreases in the formability and BHresponse of 6000-series aluminum alloy sheets due to room-temperatureaging, various proposals have been made on methods for regulating Mg—Siclusters which are formed in the sheets during room-temperature standingafter refining (after solution and quenching treatments). Among theseproposed methods is a technique in which such Mg—Si clusters arecontrolled by means of endothermic peaks and exothermic peaks of adifferential scanning thermal analysis curve (also called a differentialscanning calorimetry curve; hereinafter referred to also as DSC) of the6000-series aluminum alloy sheet.

For example, Patent Documents 1 and 2 propose that the formation amountof Mg—Si clusters that inhibit room-temperature aging and suppresslow-temperature age hardenability, in particular, Si/hole clusters(GPI), is regulated. In these techniques, for regulating the formationamount of GPI, it is regulated that the T4 material (after solutiontreatment and subsequent natural aging) gives a DSC which has noendothermic peak in the temperature range of 150-250° C., correspondingto the dissolution of GPI. In these techniques, a low-temperature heattreatment of holding at 70-150° C. for about 0.5-50 hours is performedafter a solution treatment and quenching to room temperature, in orderto inhibit or control the formation of the GPI.

Patent Document 3 proposes a 6000-series aluminum alloy sheet withexcess Si which, after a refining treatment including solution andquenching treatments of this aluminum alloy sheet, gives a DSC in whichan endothermic peak in the temperature range of 150-250° C. andcorresponds to a dissolution of Si/hole clusters (GPI) has a minusheight of 1,000 μW or less and an exothermic peak in the temperaturerange of 250-300° C. and corresponds to a precipitation of Mg/Siclusters (GPII) has a plus height of 2,000 μW or less. This aluminumalloy sheet, after having undergone room-temperature aging for at least4 months after the refining treatment, has the properties in which aproof stress is in the range of 110-160 MPa, a difference in proofstress with the one just after the refining treatment is 15 MPa or less,an elongation is 28% or greater, and a proof stress, as measured afterapplication of a 2% strain thereto and a subsequent low-temperatureaging treatment of 150° C.×20 minutes, is 180 MPa or greater.

Patent Document 4 proposes that a 6000-series aluminum alloy sheet isset to give, after a refining treatment, a DSC in which an exothermicpeak in the temperature range of 100-200° C. has a height W1 of 50 μW orlarger and a ratio of a height W2 of an exothermic peak in thetemperature range of 200 to 300° C. to the exothermic-peak height W1,(W2/W1), is 20.0 or less, in order to obtain BH response in a bakehardening treatment performed at a low temperature for a short period.

The document states that the exothermic peak W1 corresponds to theprecipitation of GP zones serving as nucleus formation sites of β″(Mg₂Si phase) in an artificial age hardening treatment, and that thehigher the W1 peak height, the more the GP zones serving as nucleusformation sites of β″ in an artificial age hardening treatment havealready been formed and secured in the sheet after refining. It statesthat as a result, the β″ grows rapidly in a bake hardening treatmentafter forming, thereby attaining an improvement in BH response. Itstates that the exothermic peak W2, on the other hand, corresponds to aprecipitation peak of the β″ itself, and that the height of thisexothermic peak W2 is made as small as possible in order to reduce theproof stress of the sheet to be formed to less than 135 MPa and tothereby ensure formability.

Patent Document 5 proposes that three exothermic-peak heights (threeportions) in a DSC in specific temperature ranges and particularlyaffect BH response are selected and regulated to enhance the BH response(bake hardenability). The three exothermic peaks are peak A at 230-270°C., peak B at 280-320° C. and peak C at 330-370° C. In the proposedmethod, the height of the peak B is regulated to 20 μW/mg or larger andthe peak ratio (A/B) and the peak ratio (C/B) are regulated to 0.45 orless and 0.6 or less, respectively, thereby attaining an increase in0.2% proof stress, through an artificial hardening treatment of 170°C.×20 minutes after application of a 2% strain, of 100 MPa or greater.

PRIOR ART DOCUMENTS Patent Documents Patent Document 1: JP-A-10-219382Patent Document 2: JP-A-2000-273567 Patent Document 3: JP-A-2003-27170Patent Document 4: JP-A-2005-139537 Patent Document 5: JP-A-2013-167004SUMMARY OF THE INVENTION Problem that the Invention is to Solve

The various outer panels for automobiles are required to attainstrain-free, beautiful curved-surface configurations and characterlines, from the standpoint of design. However, since higher-strengthaluminum alloy sheet materials are being adopted for the purpose ofweight reduction and this results in difficulties in forming, it isbecoming difficult year by year to meet such requirements. There ishence a growing desire in recent years for a high-strength aluminumalloy sheet having even better formability. However, with theabove-mentioned conventional structure controls with a DSC, it isdifficult to meet such requirements.

For example, one cause which renders high-strength aluminum alloy sheetsdifficult to apply to outer panels is the shapes peculiar to outerpanels. Recessed portions having given depths (protrudent portions,embossed portions) for attaching devices or members, such as knob mountbases, lamp mount bases and license (number plate) mount bases, or fordrawing wheel arches are partly provided to outer panels.

In the cases when such a recessed portion is press-formed together withconsecutive curved surfaces around the recessed portion shape, facestrains are prone to occur and it is difficult to attain thestrain-free, beautiful curved-surface configuration and character line.Consequently, application of high-strength aluminum alloy sheets to theouter panels has a problem in that it is necessary to obtain ahigh-strength aluminum alloy sheet which has improved formability and isinhibited from suffering face strains.

The problem concerning such face strains is not for those recessedportions (protrudent portions) but a problem common to automotive panelswhich partly have a recessed portion (protrudent portion) that maysuffer a face strain, such as a saddle-shaped portion of a door outerpanel, a vertical wall portion of a front fender, a wind corner portionof a rear fender, a character-line termination portions of a trunk lidor hood outer panel, and a root portion of a rear fender pillar.

From the standpoint of attaining improved formability for inhibiting theoccurrence of the face strains to overcome the problem described above,it is desirable that a sheet in press forming, which has undergoneroom-temperature aging after production, should have a 0.2% proof stressreduced to less than 110 MPa. However, in the cases when the proofstress in forming has been reduced as the above, it is difficult toattain a 0.2% proof stress of 170 MPa or greater after bake hardening(hereinafter also referred to as “after BH”) and to attain an increasein 0.2% proof stress through bake hardening of 70 MPa or greater. Asdescribed above, with conventional structure controls with a DSCdisclosed in Patent Documents 1 to 5, it is difficult to overcome theproblem.

The present invention has been achieved in order to overcome the problemdescribed above. An object thereof is to provide an aluminum alloy sheetwhich combines formability and bake hardenability, that is, which canhave, in automotive-panel forming, a 0.2% proof stress reduced to 110MPa or less and can have a 0.2% proof stress after BH of 170 MPa orgreater.

Means for Solving the Problem

The present inventors diligently made investigations and, as a result,have discovered that an aluminum alloy sheet which combines formabilityand bake hardenability can be obtained by adopting a specificcomposition and specific exothermic peaks in the DSC for an Al—Mg—Sialloy sheet, which contains Mg and Si. The present invention has beenthus completed.

The gist of the aluminum alloy sheet of the present invention, which isexcellent in terms of formability and bake hardenability, is an Al—Mg—Sialloy sheet containing, in terms of mass %, Mg: 0.2-1.0% and Si:0.2-1.0% and satisfying {(Mg content)+(Si content)}≤1.2%, with theremainder being Al and unavoidable impurities, in which a differentialscanning thermal analysis curve of the aluminum alloy sheet has, in atemperature range of 230-330° C., only one exothermic peak (i) or onlytwo exothermic peaks (ii) having a temperature difference between thepeaks of 50° C. or less, and in which the exothermic peak (i) or thepeak having a higher peak height of the exothermic peaks (ii) has aheight in a range of 20-50 μW/mg.

The differential thermal analysis at each of measurement portions in thesheet is performed under the same conditions including a test apparatusof DSC220G, manufactured by Seiko Instruments Inc., a referencesubstance of aluminum, a sample container made of aluminum, temperatureincrease conditions of 15° C./min, an atmosphere of argon (50 mL/min),and a sample weight of 24.5-26.5 mg. The differential thermal analysisprofile (μW) obtained is divided by the sample weight and therebynormalized (μW/mg). Thereafter, in the range of 0-100° C. in thedifferential thermal analysis profile, a region where the differentialthermal analysis profile is horizontal is taken as a reference level of0, and the height of exothermic peak from the reference level ismeasured.

The aluminum alloy sheet excellent in terms of formability and bakehardenability may further contain one element or two or more elementsselected from the group consisting of Fe: more than 0% and 0.5% or less,Mn: more than 0% and 0.3% or less, Cr: more than 0% and 0.3% or less,Zr: more than 0% and 0.1% or less, V: more than 0% and 0.1% or less, Ti:more than 0% and 0.1% or less, Cu: more than 0% and 0.5% or less, Ag:more than 0% and 0.1% or less, and Zn: more than 0% and 0.5% or less.

Effects of the Invention

According to the present invention, the contents of Mg and Si, which aremajor elements of an Al—Mg—Si alloy sheet, are regulated to berelatively low, thereby enabling a 0.2% proof stress in forming of thesheet, which has been produced and then subjected to room-temperatureaging, to be reduced to 110 MPa or less. Consequently, it can haveimproved formability when applied to automotive panels or the like,which are particularly problematic in face strains thereof, inautomotive panel structures.

In addition, the thermal properties (structure) in the DSC of thealuminum alloy sheet are regulated. As a result, an increased strengthwhich includes a 0.2% proof stress after BH of 170 MPa or greater and anincrease in 0.2% proof stress of 70 MPa or greater, which is useful asautomotive panels can be ensured. The regulation of thermal properties(structure) in the DSC provides a measure for ensuring the amount ofprecipitates which precipitate after a bake hardening treatment.

Due to such regulation of composition and structure, an aluminum alloysheet which combines formability and bake hardenability can be providedmerely with a basic composition of Al—Mg—Si alloys, without the need ofnewly adding any additive element or without the need of giving a largemodification to ordinary production processes.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a view which shows DSCs of the aluminum alloy sheets of someexamples in the Examples.

MODES FOR CARRYING OUT THE INVENTION

Modes for carrying out the present invention will be specificallyexplained below with respect to each requirement. In this description,“mass %” has the same meaning as “wt %”.

(Chemical Component Composition)

First, the chemical component composition of the Al—Mg—Si (hereinafterreferred to also as 6000-series) aluminum alloy sheet (hereinafter alsoreferred to simply as “aluminum alloy sheet”) according to the presentinvention is explained below.

The 6000-series aluminum alloy sheet targeted by the present invention,as, for example, a sheet for the automotive outer panels, is required tohave various properties such as excellent formability, BH response,strength, weldability, and corrosion resistance. Consequently, suchrequirements are also met by means of the composition. In addition, inthe present invention, the contents of Mg and Si, which are majorelements, are regulated so as to be relatively low, thereby reducing a0.2% proof stress in forming of the sheet, which has been produced andthen subjected to room-temperature aging, to 110 MPa or less. Thus, theformability into automotive panels or the like, which are particularlyproblematic in face strains thereof, in automotive panel structures, canbe improved. Simultaneously therewith, a 0.2% proof stress after bakehardening of 170 MPa or greater is rendered possible by means ofcomposition.

In order to satisfy such requirements, the aluminum alloy sheet has acomposition which contains, in terms of mass %, Mg: 0.2-1.0% and Si:0.2-1.0% and satisfies {(Mg content)+(Si content)}≤1.2%, with theremainder being Al and unavoidable impurities. In this description, allthe content indicated in % of the elements means that in mass %.Furthermore, the “-” in each content means that the content is equal toor more than the lower limit value but is equal to or less than theupper limit value.

In the present invention, elements other than Mg, Si and Al basicallyare impurities or elements which may be contained. The contents of suchother elements are the contents (permissible amounts) on levels inaccordance with the AA or JIS standards, etc., or are on levels belowsuch standards. Namely, there are cases, in the present invention also,where not only high-purity Al base metal but also 6000-series alloyscontaining elements other than Mg and Si as additive elements (alloyingelements) in large amounts, other aluminum alloy scrap materials,low-purity Al base metal, and the like are used in large quantities asmelted raw materials for the alloy, from the standpoint of resourcerecycling. In such cases, other elements shown below are inevitablyincluded in substantial amounts. Since refining performed forintentionally diminishing these elements itself leads to an increase incost, it is necessary to accept an inclusion of some degree of amount.There are content ranges which do not defeat or lessen the object oreffects of the present invention, even if included in substantialamounts.

Consequently, in the present invention, examples of the other elementswhich may be contained in the aluminum alloy include the followingelements. The permissible contents thereof are within the ranges ofequal to or less than the upper limits according to the AA or JISstandards or the like, and are as shown below.

Specifically, the aluminum alloy sheet may further contain one elementor two or more elements selected from the group consisting of Fe: 0.5%or less (exclusive of 0%), Mn: 0.3% or less (exclusive of 0%), Cr: 0.3%or less (exclusive of 0%), Zr: 0.1% or less (exclusive of 0%), V: 0.1%or less (exclusive of 0%), Ti: 0.1% or less (exclusive of 0%), Cu: 0.5%or less (exclusive of 0%), Ag: 0.1% or less (exclusive of 0%), and Zn:0.5% or less (exclusive of 0%), within those ranges.

In this description, the expression “exclusive of 0%” has the samemeaning as that the content is “higher than 0%”.

The content range of each element and the purposes and permissibleamount thereof in the 6000-series aluminum alloy sheet according to thepresent invention are explained below.

Si: 0.2-1.0%

Si, together with the Mg, is an essential element for obtaining thestrength (proof stress) required as automotive panels because it formsaging precipitates which contribute to an improvement in strength,during an artificial aging treatment such as a baking treatment, andthus exhibits an age hardenability. In the case where the content of Siis too low, the amount of aging precipitates after an artificial agingtreatment is too small, resulting in too small an increase in strengthafter baking. Meanwhile, in the case where the content of Si is toohigh, not only the strength of the sheet just after production but alsothe amount of room-temperature aging after the production are increased,resulting in too high a strength before forming. Because of this, theformability into automotive panels or the like, which are particularlyproblematic in face strains thereof, in automotive panel structures, isreduced. In addition, coarse crystals and precipitates are formed,resulting in a considerable decrease in bendability. A preferred upperlimit of the content of Si is 0.8%.

For attaining an excellent age hardenability in a baking treatmentperformed at a lower temperature for a shorter period after forming intopanels, it is preferable to employ a 6000-series aluminum alloycomposition in which Si/Mg is 1.0 or larger in terms of mass ratio sothat Si has been incorporated further excessively relative to the Mgthan in the so-called excess-Si type.

Mg: 0.2-1.0%

Mg is also an essential element for obtaining the proof stress requiredas panels, since it forms, together with the Si, aging precipitateswhich contribute to an improvement in strength, and thus exhibits an agehardenability. In the case where the content of Mg is too low, theprecipitate amount of precipitates after an artificial aging treatmentis too small, resulting in too small an increase in strength afterbaking. Meanwhile, in the case where the content of Mg is too high, notonly the strength of the sheet just after production but also the amountof room-temperature aging after the production are increased, resultingin too high a strength before forming. Because of this, the formabilityinto automotive panels or the like, which are particularly problematicin face strains thereof, in automotive panel structures, is reduced. Apreferred upper limit of the content of Mg is 0.8%.

{(Mg Content)+(Si Content)}≤1.2%

{(Mg content)+(Si content)}, which is the total content of Mg and Si, asthe structure of the 6000-series aluminum alloy sheet before forming,considerably affects exothermic peaks present in the temperature rangeof 230-330° C. in the DSC of this aluminum alloy sheet.

On the assumption that the appropriate production process which will bedescribed later is used, by regulating {(Mg content)+(Si content)} to1.2% or less, in the case where there exist only two exothermic peaks(ii) in the temperature range of 230-330° C., the difference intemperature between the peaks of the two exothermic peaks (ii) can be50° C. or less and the one having a higher peak height can have a peakheight in the range of 20-50 μW/mg. Meanwhile, in the case where thereexists only one exothermic peak (i) in that temperature range, thisexothermic peak (i) can have a height in the range of 20-50 μW/mg.

Consequently, it is preferred that {(Mg content)+(Si content)} is assmall as possible. However, since there essentially are minimumnecessary Mg and Si amounts for exhibiting basic performances as asheet, a lower limit of {(Mg content)+(Si content)} is determined by theminimum contents of these each. From this standpoint, a lower limit of{(Mg content)+(Si content)} is preferably 0.6% or higher.

Meanwhile, in the case where {(Mg content)+(Si content)} is too highabove 1.2%, it is difficult to regulate the DSC exothermic peaks so asto fall within the specified ranges, even if the appropriate productionprocess which will be described later is used. Specifically, in the casewhere there are two exothermic peaks in the temperature range of230-330° C., these two exothermic peaks cannot have a temperaturedifference between the peaks of 50° C. or less. In the case where thereis only one exothermic peak in that temperature range, this exothermicpeak cannot have a height in the range of 20-50 μW/mg. Because of this,it is difficult to attain both a reduction in strength during forming(before baking) and an enhancement in increase in strength through paintbaking. Consequently, an upper limit of {(Mg content)+(Si content)} is1.2% or less and preferably 1.0% or less.

(Differential Scanning Thermal Analysis Curve, Differential ScanningCalorimetry Curve, DSC)

The composition described above is employed. Furthermore, in the presentinvention, peaks in the DSC of the aluminum alloy sheet are regulated asa measure for ensuring the amount of precipitates which precipitateafter a bake hardening treatment, in order to ensure high strength asautomotive panels or the like. Specifically, a structure is configuredin which two exothermic peaks, which have conventionally been present inthe temperature range of 230-330° C. apart from each other, are presentso as to near to each other (with a reduced temperature difference) andto overlap each other. This makes it possible to attain a 0.2% proofstress in forming into automotive panels reduced to 110 MPa or less andto attain a 0.2% proof stress after bake hardening of 170 MPa orgreater.

Here, the differential scanning calorimetry curve (DSC) is a heatingcurve from solid phase, obtained by measuring the thermal changes duringmelting of aluminum alloy sheet after the refining treatment of thesheet, by differential thermal analysis performed under the followingconditions.

Specifically, the differential thermal analysis at each of measurementportions in the aluminum alloy sheet is performed under the sameconditions including a test apparatus of DSC220G, manufactured by SeikoInstruments Inc., a reference substance of aluminum, a sample containermade of aluminum, temperature increase conditions of 15° C./min, anatmosphere of argon (50 mL/min), and a sample weight of 24.5 to 26.5 mg.The differential thermal analysis profile (μW) obtained is divided bythe sample weight and thereby normalized (μW/mg). Thereafter, in therange of 0 to 100° C. in the differential thermal analysis profile, aregion where the differential thermal analysis profile is horizontal istaken as a reference level of 0, and the height of exothermic peak fromthe reference level is measured.

In the DSC, according to conventional techniques, there are twoexothermic peaks β″ and β′ in the range of 230-330° C., existing apartfrom each other so as to have a large temperature difference (distance)between the peaks. In the present invention, the structure of thealuminum alloy sheet has been specified so that the two exothermic peaksare located near to each other (with a reduced temperature differencetherebetween) and to overlap each other. Specifically, in a DSC of thealuminum alloy sheet, in the temperature range of 230-330° C., there isonly one exothermic peak (i) or there are only two exothermic peaks(ii), having the difference in temperature between the peaks of 50° C.or less. Moreover, the only one exothermic peak (i), or the exothermicpeak having a larger (higher) peak height of the only two exothermicpeaks (ii) has a height in the range of 20-50 μW/mg.

In 6000-series aluminum alloy sheets, various precipitate phases areyielded, depending on aging temperatures, such as clusters, GP zones,strengthening phase 1 (β″), strengthening phase 2 (β′), and equilibriumphase (Mg₂Si). It is presumed that for enhancing the strength afterbaking (artificial aging treatment), it is effective to yield β″ and β′,among those phases, during the baking. However, the 6000-series aluminumalloy sheet of the present invention, in which the contents of Mg and Sihave been regulated so as to be relatively low in order to make thesheet have, in forming after room-temperature aging, a 0.2% proof stressreduced to 110 MPa or less, considerably differs in the appearingbehavior (appearing temperature) of the strengthening phase 1 (β″) andstrengthening phase 2 (β′) upon BH (artificial aging treatment), fromordinary 6000-series aluminum alloy sheets having relatively high Mg andSi contents.

The changes in the appearing behavior of β″ and β′ upon BH (upon bakingtreatment) can be simulated with DSC. This is a base of specifying thestructure in the present invention by means of DSC.

A simulation with DSC of the appearing behavior of β″ and β′ upon BHshows that in the case of, for example, ordinary 6000-series aluminumalloy sheets having relatively high Mg and Si contents, the exothermicpeaks assigned to β″ and β′ are present more widely apart from eachother in the range of 230-330° C. More specifically, a conventionalexothermic peak assigned to β″ is mostly present around 240-260° C.,which is the lower-temperature former half of that temperature range.Meanwhile, a conventional exothermic peak assigned to β′ is presentaround 310-320° C., which is the higher-temperature latter half of thattemperature range, and they have existed in a state that the differencein temperature between the peaks of β″ and β′ has been larger than 50°C.

Such state of conventional exothermic peaks is a representative example,and that appearing behavior of the exothermic peaks varies widely, as amatter of course, depending on the composition of the sheet andproduction conditions. For example, there are cases where a DSC hasthree exothermic peaks (three portions) regarding BH response and theyare respectively called peak A at 230-270° C., peak B at 280-320° C. andpeak C at 330-370° C., as in Patent Document 5.

In contrast, when a simulation with DSC of the appearing behavior of β″and β′ upon BH is similarly made with respect to the 6000-seriesaluminum alloy sheet of the present invention, in which the contents ofMg and Si are relatively low, it can be seen that the exothermic peaksassigned to β″ and β′ are characterized in that the positions where theexothermic peaks appear (peak positions) and the distance between thepeaks (temperature difference) are nearer to each other (overlapping),as compared with those ordinary 6000-series aluminum alloy sheets. Therealso is a feature in which this phenomenon occurs as a result ofchanging the conditions for sheet production, in particular, theconditions for a preliminary aging treatment performed after solutionand quenching treatments.

In a 6000-series aluminum alloy sheet of the present invention havingrelatively low Mg and Si contents, when produced by an ordinary process,exothermic peaks of β″ and β′ exist in the wide temperature range of230-330° C. as two separate peaks, the distance between whose peaks is50° C. or larger in terms of temperature difference, like ordinary6000-series aluminum alloy sheets having relatively high Mg and Sicontents. As typical examples thereof, the DSC indicated by the brokenline shown in FIG. 1, which will be described later, and ComparativeExample 19 in Table 2 in the Examples.

On the other hand, it has been found that in the cases when a productionprocess is modified to perform the refining after rolling of the sheetso that the conditions for a preliminary aging treatment after solutionand quenching treatments are changed, the exothermic peaks of β″ and β′appear so that the peaks thereof overlap each other (are located near toeach other), with the difference in temperature between the peaks beingas small as less than 50° C.

According to the finding made by the present inventors, the appearingtemperature of the exothermic peak assigned to β″ (also called first orformer-half peak) shifts from the position (temperature) around 250-260°C. of low temperature to a position (temperature) around 270-290° C. ofhigh temperature. Meanwhile, the appearing temperature of the exothermicpeak assigned to β′ (also called second or latter-half peak) shifts fromthe position (temperature) around 300-310° C. of high temperature to aposition (temperature) around 290-300° C. of low temperature.

It has been found that in the cases when the exothermic peaks assignedto β″ and β′ have appeared so that the peaks are located near to eachother or overlap each other, with the temperature difference between thepeaks being as small as less than 50° C., then an amount ofartificial-aging precipitates which serve to enhance the proof stressafter BH can be ensured. Namely, by regulating the exothermic peaksassigned to β″ and β′ so as to be located near to each other or overlapeach other, the 0.2% proof stress in panel forming can be reduced to 110MPa or less and, simultaneously therewith, the 0.2% proof stress of thepanel after BH can be increased to 170 MPa or greater. In contrast, inthe case where those two exothermic peaks have the difference intemperature between the peaks as large as more than 50° C., thoseproperties cannot be exhibited.

One of the features of the present invention is that the state in whichthe exothermic peaks assigned to β″ and β′ overlap each other has beenspecified as above. Specifically, the 6000-series aluminum alloy sheetgives a DSC in which only two (only two in total) exothermic peaks,i.e., a lower-temperature-side exothermic peak assigned to β″ and ahigher-temperature-side exothermic peak assigned to β′, that have adifference in temperature between the peaks of 50° C. or less,preferably 30° C. or less, are present in the temperature range of230-330° C., preferably in the temperature range of 250-320° C., and inwhich the height of either exothermic peak of these, which has a larger(higher) peak height is in the range of 20-50 μW/mg. In the case wherethe lower-temperature-side exothermic peak assigned to β″ and thehigher-temperature-side exothermic peak assigned to β′ are locatednearer to each other to overlap each other so that the difference intemperature between these peaks cannot be recognized (measured), i.e.,in the case where it is deemed that there is only one so-calledsynthesized (superposed) exothermic peak in the temperature range of230-330° C., then the height of this exothermic peak is in the range of20-50 μW/mg.

In the present invention, in the case where only two exothermic peakshaving a difference in temperature between the peaks of 50° C. or less,preferably 30° C. or less, are present in the temperature range of230-330° C., preferably in the temperature range of 250-320° C., it ispreferable that the exothermic peak assigned to β″ should be presentaround 270-290° C. as a lower-temperature-side first or former-halfpeak. It is also preferable that the exothermic peak assigned to β′should be present around 290-300° C. as a higher-temperature-side secondor latter-half peak. Furthermore, the difference in temperature betweenthe peaks of these exothermic peaks is 50° C. or less, and the height ofthe exothermic peak, which has a higher peak height of these exothermicpeaks is in the range of 20-50 μW/mg. Examples thereof are the thickcontinuous line among the DSCs shown in FIG. 1, which will be describedlater, and Invention Examples 0, 1, 16, 17, 19, 21, etc. shown in Table2 in the Examples.

Meanwhile, the thin continuous line among the DSCs shown in FIG. 1,which will be described later, and Invention Examples 5, 6, 12, 15, 18,20, etc. shown in Table 2 in the Examples are the case where alower-temperature-side exothermic peak assigned to β″ and ahigher-temperature-side exothermic peak assigned to β′ more overlap eachother to render the difference in temperature between these peaksunrecognizable and, hence, there is only one synthesized exothermic peakin the temperature range of 230-330° C., preferably in the temperaturerange of 270-300° C.

Also important for ensuring the BH response is, of course, the height ofan exothermic peak which indicates the amount of artificial-agingprecipitates in BH. Namely, in the case where there are two exothermicpeaks in the temperature range of 230-330° C., the height (μW/mg) of theexothermic peak assigned to β′ (appearing around about 300° C. inInvention Examples in the Examples, which will be described later),which is the exothermic peak having a larger peak height andcontributing to BH response, is regulated so as to be in the range of20-50 μW/mg.

Meanwhile, in the case where there is only one exothermic peak in thetemperature range of 230-330° C., that is, in the case where theexothermic peak assigned to β″ (the first or former-half peak,preferably appearing around 270-290° C.) and the exothermic peakassigned to β′ (the second or latter-half peak, preferably appearingaround 290-300° C.) overlap each other to form only one synthesizedexothermic peak, the height of this exothermic peak is regulated so asto be in the range of 20-50 ρW/mg.

Thus, it is possible to reduce the proof stress in panel forming to 110MPa or lower and to attain a proof stress after BH of 170 MPa orgreater. In other words, aging precipitates of β″ and β′ which areyielded during BH can be ensured in such an amount that a proof stressafter BH of 170 MPa or greater is brought about. In the case where theheights of those exothermic peaks are smaller than the lower limit of,or are larger than the upper limit of, the range of 20-50 μW/mg, thismeans that the amount of the desired aging precipitates of such as β″and β′, which have influences on BH response through a bake hardeningtreatment is too small or too large and such precipitates are unable tobe yielded in the desired amount. Because of this, it is inevitablyimpossible to attain both a reduction in proof stress in panel formingto 110 MPa or less and a control of a proof stress after BH to 170 MPaor greater.

(Production Process)

Next, a process for producing the aluminum alloy sheet according to thepresent invention is explained. The aluminum alloy sheet according tothe present invention is produced through production steps whichthemselves are common or known, by subjecting, after casting, analuminum alloy slab having the 6000-series component composition to ahomogenizing heat treatment, hot rolling and cold rolling to obtain agiven sheet thickness, followed by a refining treatment such as asolution quenching treatment.

However, for obtaining the structure specified with a DSC according tothe present invention, during those production steps, the conditions fora preliminary aging treatment after the solution and quenchingtreatments are regulated so as to be in a preferred range, as will bedescribed later. With respect to other steps, there are preferredconditions for obtaining the structure specified with a DSC according tothe present invention. Unless such preferred conditions are employed, itis difficult to obtain the structure specified with a DSC according tothe present invention.

(Melting and Casting Cooling Rate)

First, in melting and casting steps, an aluminum alloy molten metal thathas been melted and regulated so as to have a component compositionwithin the 6000-series composition range is cast by a suitably selectedordinary melting and casting method, such as a continuous casting methodor a semi-continuous casting method (DC casting method). Here, in orderto regulate the clusters so as to be in the range specified in thepresent invention, it is preferable that the average cooling rate,during the casting, from the liquidus temperature to the solidustemperature is as high (quick) as possible at 30° C./min or greater.

In the case where such temperature (cooling rate) control in ahigh-temperature range during casting is not performed, the cooling ratein this high-temperature range is inevitably low. When an averagecooling rate in the high-temperature range is low as the above, theamount of crystals yielded coarsely in the temperature range of thishigh-temperature range is increased and also unevenness in the size andamount of the crystals along the width direction and thickness directionof the slab is increased. As a result, it is highly probable that thespecified clusters cannot be regulated so as to be in the rangesaccording to the present invention.

(Homogenizing Heat Treatment)

Next, the aluminum alloy slab obtained by casting is subjected to ahomogenizing heat treatment prior to hot rolling. The purpose of thishomogenizing heat treatment (soaking treatment) is to homogenize thestructure, that is, to eliminate segregation within the grains in thestructure of the slab. The conditions are not particularly limited solong as the purpose is achieved therewith, and the treatment may be anordinary one conducted once or in one stage.

A homogenizing heat treatment temperature is suitably selected from therange of 500° C. or more and lower than the melting point, and ahomogenizing time is suitably selected from the range of 4 hours andlonger. In the case where the homogenizing temperature is low, thesegregation within grains cannot be sufficiently eliminated, and theseact as starting points for fracture, resulting in decreases in stretchflangeability and bendability. When hot rolling is thereafter startedimmediately or when hot rolling is started after holding and cooling toan appropriate temperature, control within the number density of theclusters specified in the present invention can be achieved.

After the homogenizing heat treatment has been performed, cooling toroom temperature may be performed so that the average cooling rate inthe range of 300° C. to 500° C. is 20 to 100° C./hour, followed byreheating to 350° C. to 450° C. at an average heating rate of 20 to 100°C./hour to start hot rolling in this temperature range.

In the cases when the average cooling rate after the homogenizing heattreatment and the reheating rate conducted thereafter do not satisfythose conditions, the possibility of forming coarse Mg—Si compoundsincreases.

(Hot Rolling)

The hot rolling is constituted of a slab rough rolling step and a finishrolling step in accordance with the thickness of the plate to be rolled.In these rough rolling step and finish rolling step, rolling mills suchas a reverse type and a tandem type are suitably used.

In the cases when the hot-rolling (rough-rolling) start temperatureexceeds the solidus temperature, burning occurs and, hence, the hotrolling itself is difficult to carry out. Meanwhile, in the cases whenthe hot-rolling start temperature is lower than 350° C., the hot-rollingload is too high, rendering the hot rolling itself difficult.Consequently, the hot-rolling start temperature is preferably in therange of 350° C. to the solidus temperature, more preferably in therange of 400° C. to the solidus temperature.

(Annealing of the Hot-Rolled Plate)

Annealing (rough annealing) before cold rolling is not always necessaryfor the hot-rolled plate. However, it may be performed in order tofurther improve properties such as formability by making the grainssmaller and optimizing the texture.

(Cold Rolling)

In cold rolling, the hot-rolled sheet is rolled to produce a cold-rolledsheet (including a coil) having a desired final sheet thickness.However, for making the grains even smaller, it is desirable that thecold rolling ratio should be 60% or greater. Intermediate annealing maybe performed between cold-rolling passes for the same purpose as in therough annealing.

(Solution Treatment and Quenching Treatment)

After the cold rolling, a solution treatment is performed, followed by atreatment for quenching to room temperature. The solution and quenchingtreatments may be a heating and a cooling performed on an ordinarycontinuous heat treatment line, and are not particularly limited.However, from the standpoint of obtaining a sufficient solid-solutionamount of each element and because it is desirable that the grainsshould be finer as stated above, it is desirable that the treatmentsshould be conducted under such conditions of heating at a heating rateof 5° C./sec or greater to a solution treatment temperature which is520° C. or higher and lower than the melting temperature, and thenholding for 0.1-10 seconds.

From the standpoint of suppressing the formation of coarse intergranularcompounds that reduce the formability and hem workability, it isdesirable that the average cooling rate from the solution treatmenttemperature to the quenching stop temperature, which is roomtemperature, should be 3° C./sec or greater. In the case where theaverage rate of cooling to room temperature after the solution treatmentis too low, coarse Mg₂Si and elemental Si are yielded during thecooling, resulting in impaired formability. In addition, thesolid-solution amount after the solution treatment is reduced, resultingin a decrease in BH response. In order to secure that cooling rate,means such as air cooling with fans or water cooling with mist or sprayor by immersion, etc. and conditions therefor are selected and used forthe quenching treatment.

(Preliminary Aging Treatment: Reheating Treatment)

After having thus undergone the solution treatment and the subsequentquenching treatment to be cooled to room temperature, the cold-rolledsheet is subjected to a preliminary aging treatment (reheatingtreatment) within 1 hour. In the case where the room-temperature holdingperiod from termination of the treatment for quenching to roomtemperature to initiation of the preliminary ageing treatment(initiation of heating) is too long, clusters that are prone to dissolveupon room-temperature aging are yielded, making it impossible to formthe exothermic peaks, as a prerequisite, specified with a DSC accordingto the present invention. Consequently, the shorter the room-temperatureholding period, the better. The solution and quenching treatments andthe reheating treatment may be consecutively performed so that there issubstantially no pause therebetween, and a lower limit of the period isnot particularly determined.

In this preliminary aging treatment, it is important that periods ofholding both in the relatively higher-temperature-side range of 80-120°C. and in the relatively lower-temperature-side range of 60-40° C.should be ensured. Thus, the exothermic peaks specified with a DSCaccording to the present invention are formed.

Here, the higher-temperature-side range of 80-120° C. and thelower-temperature-side range of 60-40° C. may be divided into stages,e.g., in two stages, in terms of temperature, or may be regulated sothat the temperature changes continuously. Furthermore, the temperatureholding in the higher-temperature-side range may be a heat treatment inwhich a constant temperature within that temperature range is maintainedor in which the temperature is gradually changed within that temperaturerange by temperature increase. Meanwhile, the temperature holding in thelower-temperature-side range may be a heat treatment in which a constanttemperature within that temperature range is maintained or in which thetemperature is gradually changed within that temperature range bytemperature decrease. In short, the temperature may be continuouslychanged by temperature increase, temperature decrease (annealing), etc.,so long as the temperature is held in each of the temperature ranges forthe necessary holding period. The temperature holding in thehigher-temperature-side and in the lower-temperature-side may be a heattreatment of consecutive two stages in which the temperature is dividedinto stages, or may be heat treatment in which the holding temperatureis kept constant within each of the specified temperature ranges or maybe a continuous heat treatment in which temperature increase,temperature decrease, natural cooling, etc are suitably combined withineach of the specified temperature ranges. The cooling after thepreliminary aging treatment may be natural cooling or rapid cooling.

The period of holding in the higher-temperature-side range of 80-120° C.in the former half is preferably regulated to 5-40 hours including thetime period during which the sheet is held in the temperature range of80-120° C. in the temperature increase of the sheet. Meanwhile, theperiod of holding in the lower-temperature-side range of 60-40° C. inthe latter half is preferably regulated to 20-300 hours including theperiod of temperature decrease from the holding in thehigher-temperature-side range or the time period during which the sheetis held in the temperature range of 60-40° C. in the cooling such asnatural cooling or rapid cooling.

In the case where those temperatures are too low or those holdingperiods are too short, similar to in the case where no preliminary agingtreatment is performed, the structure according to the present inventionspecified with a DSC is less apt to be obtained, and no exothermic peakappears in the temperature range of 230-330° C. or, even if twoexothermic peaks appear, the temperature difference between the peaksexceeds 50° C. or the specified exothermic peak height exceeds 50 μW/mg.

Conversely, also in the case where those temperatures are too high orthose holding periods are too long, the structure according to thepresent invention specified with a DSC is less apt to be obtained, andno exothermic peak appears in the temperature range of 230-330° C. orthe specified exothermic peak height exceeds 50 μW/mg.

Examples

The present invention will be explained below in more detail byreference to Examples. However, the present invention should not, ofcourse, be construed as being limited by the following Examples, and canbe suitably modified unless the modifications depart from the gist ofthe present invention described hereinabove and hereinafter. All suchmodifications are included in the technical range of the presentinvention.

Examples according to the present invention are explained. 6000-seriesaluminum alloy sheets were individually produced so as to differ in thestructure specified with a DSC in the present invention, by changing theconditions for a preliminary aging treatment performed after solutionand quenching treatments. After a holding at room temperature for 30days after the production of the sheets, BH response (bakehardenability), As proof stress as an index of press formability and hemworkability as bendability are examined and evaluated.

For the individual producing, the 6000-series aluminum alloy sheetshaving the compositions shown in Table 1 was produced by variouslychanging conditions such as the temperature and holding period in thepreliminary aging treatment after the solution and quenching treatmentsas shown in Table 2. With respect to the indications of the contents ofelements within Table 1, a value of the element expressed by a blankindicates that the content is below a detection limit.

Specific conditions for aluminum alloy sheet production were as follows.Slabs of aluminum alloys respectively having the compositions shown inTable 1 were commonly produced through casting by the DC casting method.In this casting, the average rate of cooling from the liquidustemperature to the solidus temperature was set at 50° C./min in commonwith all the Examples. Subsequently, the slabs were subjected to asoaking treatment of 540° C.×6 hours, followed by initiation of hotrough rolling at that temperature, in common with all the Examples.Thereafter, they were hot-rolled, in the succeeding finish rolling, to athickness of 3.5 mm to obtain hot-rolled sheets, in common with all theExamples. The hot-rolled aluminum alloy sheets were subjected to roughannealing of 500° C.×1 minute and then to cold rolling at a processingrate of 70% without performing intermediate annealing during thecold-rolling passes, to obtain cold-rolled sheets having a thickness of1.0 mm, in common with all the Examples.

Furthermore, the cold-rolled sheets were each continuously subjected toa refining treatment (T4) with continuous type heat treatment facilitieswhile unwinding and winding each sheet, in common with all the Examples.Specifically, a solution treatment was performed by heating at anaverage rate of heating to 500° C. of 10° C./sec and holding for 5seconds after the temperature reached a target temperature of 540° C.,followed by cooling to room temperature by performing water cooling atan average cooling rate of 100° C./sec. After this cooling, apreliminary aging treatment was performed in two stages of thehigher-temperature-side range and the lower-temperature-side range,using the temperatures (° C.) and holding periods (hr) shown in Table 2.Specifically, this two-stage preliminary aging treatment was performedby holding at the given temperature for the given period by using an oilbath, as the higher-temperature-side range, and thereafter, by holdingat the given temperature for the given period by using a thermostaticoven, as the lower-temperature-side range, followed by annealing(natural cooling).

In the preliminary aging treatment, the period of holding in thehigher-temperature-side range included the time period during which thesheet was held in the temperature range of 80-120° C. in the temperatureincrease of the sheet. The period of holding in thelower-temperature-side range included the temperature decrease from theholding in the higher-temperature-side range or the time period duringwhich the sheet was held in the temperature range of 60-40° C. in thecooling by natural cooling.

From the final product sheets which each had been allowed to stand atroom temperature for 30 days after the refining treatment, test sheets(blanks) were cut out and the DSC and properties of the test sheets wereexamined and evaluated. The results thereof are shown in Table 2.

(DSC)

The structure in each of ten portions of the central portion in thesheet-thickness direction in each test sheet was examined for the DSC.In the DSC (differential scanning thermal analysis curves) of thissheet, as for the average value for these ten portions, the exothermicpeaks present in the temperature range of 230-330° C. were examined.Specifically, in the cases when two exothermic peaks were present, thedifference in temperature (° C.) between these exothermic peaks and thepeak height (μW/mg) of the exothermic peak having a higher peak heightwere determined. In the cases when only one exothermic peak was present,the height (μW/mg) of this exothermic peak was determined.

The differential thermal analysis of each of the measurement portions ineach test sheet was performed under the same conditions including a testapparatus of DSC220G, manufactured by Seiko Instruments Inc., areference substance of aluminum, a sample container made of aluminum,temperature increase conditions of 15° C./min, an atmosphere of argon(50 mL/min), and a sample weight of 24.5 to 26.5 mg. The differentialthermal analysis profile (μW) obtained was divided by the sample weightand thereby normalized (μW/mg). Thereafter, in the range of 0 to 100° C.in the differential thermal analysis profile, a region where thedifferential thermal analysis profile was horizontal was taken as areference level of 0, and the height of exothermic peak from thereference level was measured. The results thereof are shown in Tables 2and 3.

(Bake Hardenability)

The test sheets which had been allowed to stand at room temperature for30 days after the refining treatment were each examined for 0.2% proofstress (As proof stress) as a mechanical property through a tensiletest. Furthermore, these test sheets were aged at room temperature for30 days, subsequently subjected to an artificial age hardening treatmentof 170° C.×20 minutes (after BH), and then examined for 0.2% proofstress (proof stress after BH) through a tensile test, in common withthe test sheets. The BH response of each test sheet was evaluated on thebasis of the difference between these 0.2% proof stresses (increase inproof stress).

With respect to the tensile test, No. 5 specimens (25 mm×50 mmGL×sheetthickness) according to JIS Z2201 were cut out of each sample sheet toperform the tensile test at room temperature. Here, the tensiledirection of each specimen was set so as to be perpendicular to therolling direction. The tensile rate was set at 5 mm/min until the 0.2%proof stress and at 20 mm/min after the proof stress. The number N ofexaminations for mechanical property was 5, and an average valuetherefor was calculated. With respect to the specimens to be examinedfor the proof stress after BH, a 2% pre-strain as a simulation of sheetpress forming was given to the specimens by the tensile tester, followedby performing the BH treatment.

(Hem Workability)

Hem workability was evaluated only with respect to the test sheets whichhad been allowed to stand at room temperature for 30 days after therefining treatment. In the test, strip-shaped specimens having a widthof 30 mm were used and subjected to 90° bending at an inward bendingradius of 1.0 mm with a down flange. Thereafter, an inner plate having athickness of 1.0 mm was nipped, and the specimen was subjected, inorder, to pre-hem working in which the bent part was further bent inwardto approximately 130° and flat-hem working in which the bent part wasfurther bent inward to 180° and the end portion was brought into closecontact with the inner plate.

The surface state, such as the occurrence of rough surface, a minutecrack or a large crack, of the bent part (edge bent part) of the flathem was visually examined and visually evaluated on the basis of thefollowing criteria. In the following criteria, ratings of 0 to 2 are onan acceptable level, and ratings of 3 and larger are unacceptable.

0, no crack and no rough surface; 1, slight rough surface; 2, deep roughsurface; 3, minute surface crack; 4, linearly continued surface crack.

As shown by alloys Nos. 0 to 9 in Table 1 and Nos. 0, 1, 5, 6, 12, and15 to 21 in Table 2, the Invention Examples each not only have acomponent composition within the range according to the presentinvention and have been produced under conditions within preferredranges but also have undergone the refining treatment, including thepreliminary aging treatment, under conditions within preferred ranges.Because of this, these Invention Examples satisfy the DSC requirementsspecified in the present invention, as shown in Table 2. That is, thesesheets each gave a DSC which had only one or only two exothermic peaksin the temperature range of 230-330° C. and in which when only twoexothermic peaks were present, then the difference in temperaturebetween the peaks was 50° C. or less and the exothermic-peak height ofone having a higher exothermic-peak height was in the range of 20-50μW/mg. Furthermore, when only one exothermic peak was present, theheight of this exothermic peak was in the range of 20-50 μW/mg.

In Table 2, as for the peak height in the case where only two exothermicpeaks were present in the temperature range of 230-330° C., the peakappeared around 300° C. had a larger peak height than the peak appearedon the lower-temperature side, in both Invention Examples andComparative Examples. Consequently, the peak height (W/mg) of thisexothermic peak was determined.

As a result, the Invention Examples each show excellent BH responsealthough the bake hardening is performed after the refining treatmentand subsequent room-temperature aging and is a treatment conducted at alow temperature for a short period of time. Furthermore, as shown inTable 2, even after the refining treatment and subsequentroom-temperature aging, they each have a relatively low As proof stressand hence show excellent press formability into automotive panels or thelike and excellent hem workability. That is, the Invention Examples,even when having undergone an automotive-baking treatment afterroom-temperature aging, were able to exhibit not only high BH responsewith a 0.2% proof stress difference of 70 MPa or greater and a 0.2%proof stress after BH of 170 MPa or greater but also press formabilitywith an As 0.2% proof stress of 110 MPa or less and satisfactorybendability.

In contrast, Comparative Examples 2 to 4, 7 to 11, 13, and 14 in Table2, which employed alloy example 1, 2 or 3 in Table 1 like InventionExamples, each have the preliminary aging treatment conditions outsidethe preferred ranges, as shown in Table 2. As a result, they each gave aDSC which was outside the range specified in the present invention, andshow enhanced room-temperature aging and, in particular, a relativelyhigh As proof stress after 30-day room-temperature holding, as comparedwith the Invention Examples having the same alloy composition. Becauseof this, they are poor in press formability into automotive panels orthe like and in hem workability and are poor also in BH response.

In Comparative Examples 2 and 9, among these, the period from thesolution treatment and the quenching treatment to room temperature tothe preliminary aging treatment (initiation of heating) is 120 minutes,which is too long. Because of this, Mg—Si clusters that do notcontribute to strength have been yielded in a large amount. Although thetwo exothermic peaks present in the temperature range of 230-330° C.have a difference in temperature between the peaks of 50° C. or less,the exothermic-peak height exceeds 50 μW/mg.

In Comparative Example 3, the period of holding in thehigher-temperature-side range in the preliminary aging treatment is 48hours, which is too long. Because of this, the one exothermic peakpresent in the temperature range of 230-330° C. has too small a heightless than 20 μW/mg.

In Comparative Examples 4, 11 and 14, the period of holding in thelower-temperature-side range in the preliminary aging treatment is 2hours, which is too short. Because of this, although the two exothermicpeaks present in the temperature range of 230-330° C. have a differencein temperature between the peaks of 50° C. or less, the exothermic-peakheight exceeds 50 μW/mg, or in the case where one exothermic peak ispresent in the temperature range of 230−330° C., this exothermic peakhas a height exceeding 50 μW/mg.

In Comparative Examples 10 and 13, the period of holding in thehigher-temperature-side range in the preliminary aging treatment is 2hours, which is too short. Because of this, in the case where oneexothermic peak is present in the temperature range of 230-330° C., thisexothermic peak has a height exceeding 50 μW/mg.

In Comparative Example 7, the temperature in the higher-temperature-siderange in the preliminary aging treatment is 70° C., which is too low.Because of this, although the two exothermic peaks present in thetemperature range of 230-330° C. have a difference in temperaturebetween the peaks of 50° C. or less, the higher exothermic peak has aheight exceeding 50 μW/mg.

In Comparative Example 8, the temperature in the higher-temperature-siderange in the preliminary aging treatment is 130° C., which is too high.Because of this, in the case where one exothermic peak is present in thetemperature range of 230-330° C., this exothermic peak has a height lessthan 20 μW/mg.

Comparative Examples 22 to 30 in Table 2 have been produced underpreferred conditions, including the conditions for the preliminary agingtreatment. However, since they employed alloys Nos. 10 to 18 shown inTable 1, the contents of Mg and Si, which are essential elements,therein are outside the ranges according to the present invention or thecontent of impurity elements therein is too high. Because of this, theseComparative Examples 22 to 30 each show, in particular, a relatively toohigh As proof stress after 30-day room-temperature holding as comparedwith the Invention Examples, as shown in Table 2. They hence are poor inpress formability into automotive panels or the like and in hemworkability or are poor in BH response. The compositions of ComparativeExamples 22 to 30 are described in detail below.

Comparative Example 22 is alloy 10 shown in Table 1, in which the Sicontent is too low.

Comparative Example 23 is alloy 12 shown in Table 1, in which the Mg+Sicontent is too high.

Comparative Example 24 is alloy 11 shown in Table 1, in which the Sicontent is too high and the Mg+Si content is too high.

Comparative Example 25 is alloy 13 shown in Table 1, in which the Fecontent is too high.

Comparative Example 26 is alloy 14 shown in Table 1, in which the Mncontent is too high.

Comparative Example 27 is alloy 15 shown in Table 1, in which the Cr andTi contents are too high.

Comparative Example 28 is alloy 16 shown in Table 1, in which the Cucontent is too high.

Comparative Example 29 is alloy 17 shown in Table 1, in which the Zncontent is too high.

Comparative Example 30 is alloy 18 shown in Table 1, in which the Zr andV contents are too high.

DSCs selected from those of the Invention Examples and ComparativeExamples are shown in FIG. 1. In FIG. 1, the thick continuous lineindicates Invention Example 1, the thin continuous line indicatesInvention Example 12 and the broken line indicates Comparative Example23.

In the DSC of Invention Example 1, a first exothermic peak of β″ appearsaround 270° C. and a second exothermic peak of β′ appears around 300° C.near the first peak, and the difference in temperature between thesepeaks is 27° C. as shown in Table 2, which is 50° C. or less asspecified.

In the DSC of Invention Example 12, a first exothermic peak of β″ and asecond exothermic peak of β′ overlap each other to form one synthesizedpeak. This synthesized peak appears around 290° C. and, as shown inTable 2, has a peak height of 35.9 μW/mg, which is in the range of 20-50μW/mg.

In contrast, in the DSC of Comparative Example 23, a first exothermicpeak of β″ appears around 260° C. and a second exothermic peak of β′appears around 310° C., and the difference in temperature between thesepeaks is 53° C. as shown in Table 2, which exceeds the specifiedtemperature of 50° C.

Those results of the Examples support that, for improving formabilityand BH response after room-temperature aging, it is necessary that allthe requirements concerning composition and DSC specified in the presentinvention should be satisfied.

TABLE 1 Alloy Chemical components of Al—Mg—Si alloy sheet (mass %;remainder, Al) No. Mg Si Mg + Si Fe Cu Mn Cr Zr V Ti Zn Ag 0 0.40 0.601.00 1 0.40 0.60 1.00 0.4 2 0.32 0.65 0.97 0.2 0.12 3 0.34 0.58 0.92 0.20.12 0.05 4 0.38 0.45 0.83 0.2 0.3 5 0.48 0.52 1.00 0.2 0.2 6 0.54 0.450.99 0.2 0.05 0.06 7 0.28 0.67 0.95 0.2 0.07 0.07 8 0.36 0.49 0.85 0.20.08 0.4 9 0.54 0.61 1.15 0.2 0.2 10 0.66 0.15 0.81 0.2 11 0.45 1.031.48 0.2 12 0.40 0.91 1.31 0.2 13 0.38 0.66 1.04 0.71 14 0.65 0.41 1.060.2 0.72 0.01 15 0.35 0.80 1.15 0.2 0.4 0.13 16 0.41 0.62 1.03 0.2 0.8817 0.31 0.58 0.89 0.2 0.95 18 0.36 0.72 1.08 0.2 0.4 0.4

TABLE 2 Preliminary aging treatment Higher-temperature-Lower-temperature- Required side range side range period to (80-120° C.)(60-40° C.) preliminary Holding Holding Alloy No. aging Temperatureperiod Temperature period Classification No. in Table 1 min ° C. hr ° C.hr Inv. Ex. 0 0 5 100 12 50 24 Inv. Ex. 1 1 5 100 12 50 24 Com. Ex. 2 1120 100 12 50 24 Com. Ex. 3 1 5 100 48 50 24 Com. Ex. 4 1 5 100 12 50 2Inv. Ex. 5 2 5 100 12 50 24 Inv. Ex. 6 2 5 110 5 50 24 Com. Ex. 7 2 5 7012 50 24 Com. Ex. 8 2 5 130 12 50 24 Com. Ex. 9 2 120 100 12 50 24 Com.Ex. 10 2 5 100 2 50 24 Com. Ex. 11 2 5 100 12 50 2 Inv. Ex. 12 3 5 10012 50 240 Com. Ex. 13 3 5 100 2 50 240 Com. Ex. 14 3 5 100 12 50 2 Inv.Ex. 15 4 5 100 12 50 24 Inv. Ex. 16 4 5 80 30 50 24 Inv. Ex. 17 5 5 9012 50 24 Inv. Ex. 18 6 15 100 12 50 24 Inv. Ex. 19 7 5 100 20 50 24 Inv.Ex. 20 8 5 100 12 40 240 Inv. Ex. 21 9 5 90 12 60 24 Com. Ex. 22 10 5100 12 50 24 Com. Ex. 23 12 5 100 12 50 24 Com. Ex. 24 11 5 100 12 50 24Com. Ex. 25 13 5 100 12 50 24 Com. Ex. 26 14 5 100 12 50 24 Com. Ex. 2715 5 100 12 50 24 Com. Ex. 28 16 5 100 12 50 24 Com. Ex. 29 17 5 100 1250 74 Com. Ex. 30 18 5 100 12 50 24 Structure of aluminum alloy sheetafter 30-day room-temperature holding Exothermic peaks at 230-330° C. indifferential scanning thermal analysis curve Height of First-peakSecond-peak Peak Alloy No. Number higher peak temperature temperaturetemperature Classification No. in Table 1 of peaks μW/mg ° C. ° C.difference Inv. Ex. 0 0 2 40.5 273 301 28 Inv. Ex. 1 1 2 41.9 273 300 27Com. Ex. 2 1 2 63.4 268 294 26 Com. Ex. 3 1 1 16.8 295 — — Com. Ex. 4 12 54.8 271 297 26 Inv. Ex. 5 2 1 33.6 290 — — Inv. Ex. 6 2 1 28.1 291 —— Com. Ex. 7 2 2 56.8 272 299 27 Com. Ex. 8 2 1 12.2 295 — — Com. Ex. 92 2 54.5 271 301 30 Com. Ex. 10 2 1 54.4 286 — — Com. Ex. 11 2 1 52.1296 — — Inv. Ex. 12 3 1 35.9 290 — — Com. Ex. 13 3 1 57.2 287 — — Com.Ex. 14 3 1 54.7 295 — — Inv. Ex. 15 4 1 35.6 295 — — Inv. Ex. 16 4 242.1 270 299 29 Inv. Ex. 17 5 2 40.1 274 301 27 Inv. Ex. 18 6 1 43.7 290— — Inv. Ex. 19 7 2 27.5 273 301 28 Inv. Ex. 20 8 1 34.8 292 — — Inv.Ex. 21 9 2 38.2 271 297 26 Com. Ex. 22 10 1 10.4 296 — — Com. Ex. 23 122 56.0 258 311 53 Com. Ex. 24 11 2 38.7 258 312 54 Com. Ex. 25 13 2 43.1271 298 27 Com. Ex. 26 14 1 35.8 291 — — Com. Ex. 27 15 2 44.5 269 29627 Com. Ex. 28 16 2 39.2 273 300 27 Com. Ex. 29 17 1 38.3 290 — — Com.Ex. 30 18 2 40.2 271 298 27 Properties of aluminum alloy after 30-dayroom-temperature holding As 0.2% 0.2% proof Proof stress Alloy No. proofstress stress after BH increase Hem Classification No. in Table 1 MPaMPa MPa workability Inv. Ex. 0 0 105 195 90 2 Inv. Ex. 1 1 103 195 92 2Com. Ex. 2 1 108 162 54 2 Com. Ex. 3 1 123 211 88 3 Com. Ex. 4 1 88 16678 1 Inv. Ex. 5 2 98 182 84 1 Inv. Ex. 6 2 107 185 78 2 Com. Ex. 7 2 105166 61 2 Com. Ex. 8 2 136 184 48 3 Com. Ex. 9 2 103 154 51 1 Com. Ex. 102 85 161 76 1 Com. Ex. 11 2 87 163 76 1 Inv. Ex. 12 3 106 184 78 2 Com.Ex. 13 3 102 166 64 2 Com. Ex. 14 3 86 166 80 1 Inv. Ex. 15 4 107 179 722 Inv. Ex. 16 4 105 182 77 2 Inv. Ex. 17 5 98 175 77 1 Inv. Ex. 18 6 102176 74 1 Inv. Ex. 19 7 108 194 86 2 Inv. Ex. 20 8 106 179 73 2 Inv. Ex.21 9 105 187 82 1 Com. Ex. 22 10 71 114 43 1 Com. Ex. 23 12 137 249 1123 Com. Ex. 24 11 146 249 103 4 Com. Ex. 25 13 107 191 84 4 Com. Ex. 2614 121 202 81 4 Com. Ex. 27 15 118 211 93 4 Com. Ex. 28 16 132 223 91 3Com. Ex. 29 17 102 180 78 4 Com. Ex. 30 18 117 201 84 4

While the present invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope of the presentinvention. This application is based on a Japanese patent applicationfiled on Mar. 31, 2014 (Application No. 2014-074044), the contentsthereof being incorporated herein by reference.

INDUSTRIAL APPLICABILITY

According to the present invention, it is possible to provide6000-series aluminum alloy sheets which combine BH response andformability after room-temperature aging. As a result, the 6000-seriesaluminum alloy sheets are usable in applications extended to automotivepanels, in particular, outer panels in which problems may ariseconcerning the design of beautiful curved-surface configurations,character lines, etc.

1. An aluminum alloy sheet excellent in terms of formability and bakehardenability, which is an Al—Mg—Si alloy sheet comprising, in terms ofmass %, Mg: 0.2-1.0% and Si: 0.2-1.0% and satisfying {(Mg content)+(Sicontent)}≤1.2%, with the remainder being Al and unavoidable impurities,wherein a differential scanning thermal analysis curve of the aluminumalloy sheet has, in a temperature range of 230-330° C., only oneexothermic peak (i) or only two exothermic peaks (ii) having atemperature difference between the peaks of 50° C. or less, and whereinthe exothermic peak (i) or the peak having a higher peak height of theexothermic peaks (ii) has a height in a range of 20-50 μW/mg.
 2. Thealuminum alloy sheet excellent in terms of formability and bakehardenability according to claim 1, further comprising one element ortwo or more elements selected from the group consisting of Fe: more than0% and 0.5% or less, Mn: more than 0% and 0.3% or less, Cr: more than 0%and 0.3% or less, Zr: more than 0% and 0.1% or less, V: more than 0% and0.1% or less, Ti: more than 0% and 0.1% or less, Cu: more than 0% and0.5% or less, Ag: more than 0% and 0.1% or less, and Zn: more than 0%and 0.5% or less.